Wednesday, January 16, 2019

Disentangling California Droughts

Devastating droughts are a great concern. Droughts disrupt ecosystems, agriculture, and drinking water supplies. Contrary to headlines suggesting we have only 12 years before descending into climate hell with more severe droughts, historically, Californians are not experiencing more severe droughts. Despite low stream flows and withering plants, there’s no agreement on how to best define drought. Different methods suggest different severities for the same drought. Thus, the Intergovernmental Panel on Climate Change’s recent assessment, downgraded their ability to detect the causes of drought to “low confidence”.

Ocean circulation determines how much rain reaches the land. Each summer, California naturally experiences months of drought because storms carrying ocean moisture are blocked. Every few years, a rainy El Niño year alternates with drought producing La Niñas. But 20 years of more frequent La Niñas can cause 20 years of drought. To address natural precipitation shifts, California constructed ~1400 dams, storing water during wet years that can be released during drought years. Yosemite’s Hetch Hetchy reservoir supplies about 25% of San Francisco’s drinking water and 17% of its electricity. Misguided attempts to remove its dam would be disastrous for humans with scant environmental benefits.

NOAA scientists analyzed California’s 2011-2014 drought concluding it was dominated by a La Niña and natural variability. In contrast, their models suggested any greenhouse contribution was “very small”. Similarly, drought-sensitive tree rings suggested the extremely low precipitation was not unprecedented nor “outside the range of natural variability”. For 1200 years, extremely low rainfall happens a few times every century. 

However, because higher temperatures can theoretically increase evaporation and dry the land, some researchers define drought by calculating the Palmer Drought Severity Index (PDSI). Despite using the same tree rings, the PDSI transformed a natural California drought into the worst in 1200 years, evoking global warming fears. 

What to trust? 

Most scientists agree the PDSI is biased towards worse droughts, because it assumes higher temperatures always dry the land. However, the opposite is also true! Without moisture to absorb heat, drier conditions produce higher temperatures.  Studies using more accurate measurements than the PDSI find no increase in global droughts. 

Before significant COwarming was possible, Dust Bowl years from 1928-1939 and the 1950s drought were the most severe 20thcentury American droughts. La Niña-like ocean temperatures blocked rain storms and triggered the Dust Bowl while plowing up native grasses made it worse. More concerning is 2 century-long megadroughts between 900 AD and 1400 AD. Trying to survive increasing dryness Native Americans created dams and irrigation canals. But those droughts finally led to the demise of once thriving Pueblo Cultures such as Mesa Verde.  

Will our modern water infrastructure protect us if drought history repeats? 

Reducing our carbon foot print or whacky plans to shade the earth from the sun to lower global temperatures will have no effect. Lower temperatures may in fact increase major droughts. Droughts during the 1750s, 1820s, and 1850s-1860s were similar to the 1950s. During the cool 1500s, the southwestern United States and Mexico suffered decades long droughts of “epic proportions”.

Coincident with the Pueblo Culture’s demise, drought is detected in sediments of San Francisco Bay. Droughts reduce stream flows that normally flush the bay, allowing salty ocean water to encroach deeper into the Bay’s delta. Past droughts caused the Bay’s Suisun Marsh to become 40% saltier. Suisun Marsh is now considered the only sustainable habitat for a critically endangered fish, the Delta Smelt. The current theory for the Delta Smelt’s demise is agricultural diversions of freshwater raised salinity to intolerable levels. That perceived competition for freshwater has pitted farmers against efforts to save the smelt.  Learning how the smelt survived a thousand years of much higher salinity might provide a win-win solution. 

Agricultural and urban needs also compete with salmon survival. One promising win-win solution is having juvenile salmon develop in irrigated rice fields after hatching. Experiments show young salmon grow much bigger in rice fields. Additionally, low stream flows hamper salmon migration. But when enough water is naturally stored as groundwater, seasonal groundwater release can maintain adequate summer stream flows.  Unfortunately, landscape changes have caused stream channels to cut downwards, draining local groundwater and drying the land. Restoring streams and groundwater would provide great benefits.

During my research in the Sierra Nevada, a meadow we were monitoring began to dry; willows died, and bird populations crashed. Many suggested it was just what global warming models predict. However, we determined a railroad track built over 100 years ago had caused the meadow’s stream channel to cut downwards, draining its groundwater. I initiated a watershed restoration. Vegetation quickly recovered, and wildlife increased. Despite California’s years of extreme drought, the restored meadow remained wetter than it had before restoration and before the drought. 

So, I warn: knee-jerk reactions simply blaming climate change for devastating dryness, blind us to real causes and real environmental solutions.


  1. Nice article as well as whole site.Thanks for sharing.

  2. One of the most worrying things in global culture right now is wilfully ignoring climatic events of more than thirty years ago. My country, UK! had a significant drought and heat wave in 2018. The febrile and excitable U35s genuinely believed this was unheard of. I grew up in the 1970s and summer 1976 was entirely similar, although heavy rains started earlier that year.

    The Mammoth Ski area website has monthly snowfall data going back to 1969 and it is quite clear that, far from a drought occurring in that part of the Sierra, average snowfall since 2000 is higher than during the late 20th century. You would never know that watching the TV. By far the worst snowfall year was in the 1970s. And the two highest, well over 600 inches, have been since 2000.

    So it is not the precipitation that is the problem, it is the storage, the exposure to evaporation, the usage and the wastage of water that are the issues at hand.